Granulosa cells in a mature ovarian follicle have an abundance of LH/hCG receptors that respond rapidly to an ovulatory surge in gonadotropins. Within minutes, membrane signal transduction sets in motion metabolic changes that lead to follicular rupture. This study provides evidence that the initial ovarian response to such an ovulatory stimulus includes induction of the immediate-early transcription factor gene for early growth response protein-1 (Egr-1). Immature Wistar rats were primed with 10 IU equine CG (eCG), sc, and 48 h later the 12-h ovulatory process was initiated by 10 IU hCG, sc. Ovarian RNA was extracted at 0, 0.5, 1, 2, 4, 8, 12, and 24 h after the primed animals were injected with hCG. The RNA extracts were used for RT-PCR differential display for random detection of gene expression in the stimulated ovarian tissue. Northern analysis of one of the differentially amplified complementary DNAs confirmed that it was part of a gene that was significantly up-regulated within 1 h after the ovaries had been stimulated by hCG. Maximum transcription was at 4 h after hCG, and expression declined to 0 h control levels by 24 h after hCG. Subcloning and sequence analysis revealed that the complementary DNA matched the gene for Egr-1. In situ hybridization indicated that the Egr-1 messenger RNA was in the granulosa layer of mature follicles. Western blotting confirmed the temporal pattern of Egr-1 expression detected by differential display, Northern analysis and in situ hybridization. The Egr-1 protein is approximately 84 kDa. In conclusion, the data show that expression of the zinc finger transcription factor Egr-1 is an early event in the cascade of inflammatory-like changes that occur in an ovulatory follicle in response to a trophic hormone.